Method for controlling a modem connection in the event of disturbance

ABSTRACT

A method for controlling a connection, especially a wireless connection, between two modems in the event of disturbance, which includes initiating a so-called retrain procedure in the event of disturbance when either one of the modems receives a signal of unsatisfactory quality. In the retrain procedure, the modems transmit predetermined retrain signals to each other in order to achieve synchronism between them. For reliable and rapid disconnection of a connection, the progress of the retrain procedure is monitored, and every time the procedure is stuck in a state other than a data transmission state for a period of time longer than a predetermined limit, the modem is commanded to initiate a repetition of the start-up procedure by means of which the connection between the modems was originally established.

This application claims benefit of international applicationPCT/F194/00375 filed Aug. 26, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling a connection,especially a wireless connection, between two modems in the event ofdisturbance, this method comprising initiating a so-called retrainprocedure in the event of disturbance when either one of the modemsreceives a signal of unsatisfactory quality, in which retrain procedurethe modems transmit predetermined retrain signals to each other in orderto achieve synchronism between them. The term wireless connection isused herein to refer to a connection implemented in such a manner thatit is at least partly wireless.

A wireless radio network is originally intended for transmission ofspeech but it is also suitable for transmission of data. Datatransmission in a radio network is carried out by means of a modem in aterminal and a mobile phone, e.g. a cellular phone. It is possible touse, for example, a modem positioned in a portable computer or a modemdesigned for a conventional telephone network. The modem is connected,for example, to a cellular phone by a separate adapter, which, forinstance, adjusts the transmission level.

FIG. 1 of the attached drawings is a block diagram of equipment, knownper se, for transmitting data between a computer 11 (typically aportable computer) or a terminal with a similar function and anotherterminal (not shown) connected to it through a radio network RN. Theequipment comprises a modem 12, which is connected to the computer 11and, via the above-mentioned adaptor 19, to a mobile phone 13. The modem12 comprises an interface unit 14 (e.g. conventional RS-232 serialinterface), to which the terminal 11 is connected, an error correctionunit 16 connected to the interface unit, a data pump 17 connected to theerror correction unit, a line interface unit 18 connected to the datapump 17, and a control processor 15 for controlling the operation of themodem. The actual modem part consists of the data pump 17, whichmodulates digital signals supplied from the terminal 11 to a formsuitable for an analog telephone network and demodulates signalssupplied from the telephone network to a form suitable for the terminalequipment. The data pump 17 is connected to the telephone network(adapter) through the analog line interface unit 18. The controlprocessor 15 of the modem controls the data pump and performs errorcorrection and all other functions of the modem, such as communicationwith the computer through the interface unit 14. The error correctionunit 16 receives data transmitted through the interface unit, packs thedata in frames of a fixed size, and transmits the frame through the datapump. At the reception end, the data pump 17 converts the receivedanalog signal into digital form, whereafter the receiving errorcorrection unit examines the received frame and detects any possibleerrors, which are corrected by means of retransmission.

In view of data transmission in a mobile telephone network, the mostsignificant component of the modem is the data pump 17. The CCITT (atpresent the ITU-T) has defined a number of modulation standards ofdifferent rates for use in a general switched telephone network. Thesestandards are illustrated in the following table.

    ______________________________________                       Modula-   Carrier  Symbol    Recom-    Rate     tion      frequencies                                          rate    mendation (bit/s)  method    (Hz)     (1/s)    ______________________________________    V.21      0-300    FSK       1080/1750                                          300    V.23      1200/75  FSK       1700/420 1200/75    V.22      600      QAM       1200/2400                                          600              1200    V.22 bis  1200     QAM       1200/2400                                          600              2400    V.32      4800     QAM and   1800     2400              9600     TCM    V.32 bis  4800     QAM and   1800     2400              7200     TCM              9600              12000              14400    ______________________________________

The abbreviations used in the third column of the table above have thefollowing meanings: FSK=Frequency Shift Keying, QAM=Quadrature AmplitudeModulation, and TCM=Trellis Code Modulation.

As appears from the table, the modems according to recommendation V.32bis offer a high rate and are thus a reasonable option even in radionetworks. A V.32 bis connection is established between modems by astart-up procedure described on page 10 of recommendation V.32 bis.There are different procedures for the calling and the answering channel(modem). The establishment of the connection is described more closelyin the above-mentioned recommendation, pages 14 to 16, which arereferred to for a more detailed description. In practice, the start-upand the retrain procedure are almost identical, as the retrain procedureforms the synchronization part of the start-up procedure.

If the modems lose synchronism during the connection, the so-calledretrain procedure, i.e. resynchronization of the modems, is carried out.The operating principle of the retrain procedure is illustrated in thefigure on page 11 of the above-mentioned recommendation. A more detaileddescription of the retrain procedure is given on page 16 of theabove-mentioned recommendation.

In a radio network there are often situations where modems losesynchronism, and the above-mentioned retrain procedure must be carriedout. Such situations may occur in a wireless network, such as a cellularnetwork, for the following reasons:

Synchronism is lost on account of change of base stations.

Noise is suddenly present on the connection, which prevents data frombeing transmitted at the current rate.

A cellular connection from a moving car brings about changes in thefield, and radio waves reflected from buildings cause interference.

The quality of the line is low. (Operations in cellular networks arealways performed within the area of a certain base station. The qualityof the line is high near the base station but gets lower as the distancefrom the base station grows. In the area where base stations are changedthe quality is usually low.)

The internal signalling (change of base stations, transmission level,field strength) carried out on a speech channel of a cellular networkcauses breaks in data transmission. Depending on the duration andfrequency of the signalling, unnecessary resynchronizations, whichweaken the performance, may take place on the data connection.

With a moving car, there are always shadow regions, in which theconnection does not work in practice. The connection is, however,restored when the shadow region is left behind. A normal data connectionis disconnected at such locations unless) special measures are taken toprevent it. A shadow region may also occur, for instance, at trafficlights, in which case the car may remain in the shadow region for rathera long time, even dozens of seconds.

The (wireless) network disconnects the connection.

In a normal telephone network the retrain procedure usually succeeds,and the modems return to the data transmission state. In a radionetwork, however, the quality of the connection varies, which causesproblems. Modems designed for use in a normal telephone network usuallydisconnect the connection after two or three retrain attempts. In amobile telephone network it is essential that the connection is notdisconnected on account of disturbances in the network. In a normalcase, the connection is disconnected by means of LINK DISCONNECTmessages from a V.42 or an MNP error corrector or a GSTN CLEARDOWNsequence defined in recommendations V.32 and V.32 bis. If the (wireless)network disconnects the connection, the modems proceed to the retrainstate and after a given time disconnect the connection as the retrainprocedure does not succeed (the far end is no longer on the line). Thegiven time must be extended even to several minutes so as to preventunnecessary disconnection of the connection.

In a mobile telephone network, the change of base stations or the shadowregions due to the environment may thus lead to loss of synchronismbetween data pumps, wherefore the data pumps start the retrain procedurein order to be able to return to the data transmission state.Disturbances in the network may, however, lead to failure of the retrainprocedure, in which case the procedure is started over again. When thequality of the connection is improved, the retrain procedure usuallysucceeds, and the modems return to the data transmission state. Veryoften, however, the retrain procedure does not succeed for someunaccountable reason no matter how high the quality of the mobiletelephone connection is. In that event, there is no other alternativethan to disconnect the connection and to establish it all over again.

As appears from the above, the problems caused in data transmission bywireless connections are such that the current methods do not provide areliable and sufficiently rapid way to maintain or disconnect aconnection.

SUMMARY OF THE INVENTION

The object of the present invention is to obviate this drawback. This isachieved with a method according to the invention, which ischaracterized in that the progress of the retrain procedure ismonitored, and every time the procedure is stuck in its internal state(i.e. an initial state, or a state other than a data transmission state)for a period of time longer than a predetermined limit, the modem iscommanded to initiate a start-up procedure known per se, by means ofwhich the connection between the modems was originally established.

Instead of establishing a completely new connection to the other endevery time the retrain procedure fails, it is the idea of the inventionto attempt to establish only a new data connection without disconnectingthe actual telephone connection if the retrain procedure indicates thatthe connection as such is in order.

The solution according to the invention allows the disconnection time tobe considerably shortened. If the entire telephone connection isdisconnected, the retrain procedure cannot proceed, wherefore theconnection can be disconnected in a clearly shorter time than before,e.g. after 60 s. If the retrain procedure proceeds, it is obvious thatthe connection as such is in order but its quality is temporarilyunsatisfactory, wherefore the connection is not disconnected butreestablishment of only the data transmission connection is attempted.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail byway of example and with reference to FIG. 2 in the accompanyingdrawings, in which:

FIG. 1 is a block diagram of equipment for implementing a wireless modemconnection, and

FIG. 2 is a state transition diagram illustrating the method accordingto the invention.

DETAILED DESCRIPTION

When the modems proceed to the retrain state after either of them hasdetected that the quality of the received signal is not satisfactory,the data connection is disconnected and the data received from the errorcorrector has no significance. In this case, the decision concerningdisconnection of the connection must be made on the basis of dataobtained from the data pump. The most reliable data on the state of theconnection is obtained by monitoring the progress of the retrainprocedure in detail. FIG. 2 illustrates more closely the principle ofthe monitoring process according to the invention.

If the retrain procedure does not proceed in the normal manner (and themodems are thus not able to return to the data transmission state),there are two alternative ways of proceeding:

1. The retrain procedure does not proceed at all.

2. The retrain procedure proceeds but not to the end; instead, it alwaysreturns to the initial state.

In the case of alternative 1, the modem does not hear the modem of theother end, wherefore no internal transitions in the retrain proceduretake place (cf. CCITT (at present the ITU-T) recommendation V.32 bis). Areason for this may be that (a) the physical line (telephone connection)is disconnected, or (b) the data pump of the modem is somehow "stuck" inone state. The situation can be checked by commanding the modem to startthe above-mentioned start-up procedure, by which the connection betweenthe modems was originally established.

The following timers and counters are used for monitoring the retrainprocedure:

RETRAIN₋₋ TIMER=a timer which measures the total time the modem hasremained in the retrain state,

RETRAIN₋₋ IDLE TIMER=a timer which measures the time the modem hasremained in the initial state of the retrain procedure at a time(between the start-up procedures),

RETRAIN₋₋ TOTAL₋₋ IDLE₋₋ TIMER=a timer which measures the total time themodem has remained in the initial state of the retrain procedure,

RETRAIN₋₋ COUNT=a counter which counts the number of retrain attempts(an attempt when the retrain procedure proceeds from its initial stateto another one of its internal states),

START-UP₋₋ COUNT=a counter which counts the number of start-upprocedures, i.e. the number of the attempts to re-establish the dataconnection of the entire physical connection.

When a transition to the retrain procedure is detected, all theabove-mentioned timers and counters are reset (step 21, FIG. 2). In theinitial state 22 of the retrain procedure, the calling modem (ORG)transmits the AA segment according to recommendation V.32 bis and theanswering modem (ANS) transmits the AC segment. From the initial stateit is possible to proceed only to two different states: either theretrain procedure starts to proceed, in which case it proceeds to state23 (another internal state of the retrain procedure), or a process fordisconnecting the connection is started and the procedure proceeds tothe disconnection state 24.

If it is detected that the retrain procedure starts to proceed, thecounter RETRAIN₋₋ COUNT is incremented by one. The detection may bebased, for example, on the fact that the calling modem ORG stopstransmitting the AA segment (ORG≠AA) or the answering modem ANS stopstransmitting the AC segment (without that the state 24 for disconnectingthe connection is proceeded to). If the retrain procedure proceeds inthe normal manner, it proceeds thereafter to the data transmission state25, whereby data transmission between the modems can be continued again.(In FIG. 2, the events that bring about each transition are indicatedabove the horizontal line beside each transition, whereas the eventsfollowing the transition are indicated below the line.)

If the retrain procedure does not proceed to the end, the modem returnsto the initial state 22 at some step of the retrain procedure. This canbe detected, for example, when the calling modem (ORG) starts again totransmit the AA segment and the answering modem (ANS) the AC segment(ORG=AA or ANS=AC). In this case, the timers RETRAIN₋₋ IDLE₋₋ TIMER andRETRAIN₋₋ TOTAL₋₋ IDLE₋₋ TIMER are reset (step 23a).

If it is detected that the retrain procedure does not proceed from theinitial state, the modem is commanded to start the start-up procedureagain. This is effected by loading those sequences (commands) whichdenote a transition to the start-up procedure into the data pump of themodem. Such a "kick" is performed every time the RETRAIN₋₋ IDLE₋₋ TIMERhas exceeded its limit, which may be e.g. 15 seconds as shown in FIG. 2.At the same time the counter START-UP₋₋ COUNT is incremented by one, andthe RETRAIN₋₋ IDLE₋₋ TIMER is reset for new measurement. As the start-upand the retrain procedure are in practice identical for the most part(they contain identical synchronization parts, but the start-upprocedure further includes listening to the answer tone, which startsthe procedure, cf. the above-mentioned recommendation), the starting upof the start-up procedure is indicated in FIG. 2 with an arrow 22areturning to the retrain initial state 22.

If, however, the total time the modem has remained in the retrain stateor in the retrain initial state 22 becomes too long or the number ofabove-mentioned "kicks" or retrain attempts becomes too high, thedisconnection state 24 is proceeded to, i.e. the process fordisconnecting the connection is started in a manner known per se(defined in the recommendations). The limits which start the transitionto the disconnection state may be, for example, as follows:

RETRAIN₋₋ TIMER=300 seconds,

RETRAIN₋₋ TOTAL₋₋ IDLE₋₋ TIMER=60 seconds,

RETRAIN₋₋ COUNT=20,

START-UP₋₋ COUNT=10.

In principle, it should be possible to carry out the above-mentionedprocess in both the calling and the answering modem, as either one ofthem may be "stuck" in one state. However, depending on the practicalrealization, it may also be sufficient if the method according to theinvention is implemented only in the answering modem.

The method according to the invention can be implemented quite easily inthe equipment according to FIG. 1 by providing the equipment with thecounters and timers described above. The other necessary modificationsconcern only the software of the modem.

Although the invention has been described above with reference to theexamples illustrated in the accompanying drawings, it is obvious thatthe invention is not limited to the examples but can be modified withinthe scope of the inventive concept disclosed above and in the appendedclaims. For instance, although the invention is primarily intended forwireless transmission connections, the same principle can also be usedfor connections of a fixed telephone network. In addition, the modems towhich the present method is applied do not necessarily have to be modemsaccording to recommendation V.32 bis, but it is also possible to usemodems according to other compatible, previous and futurerecommendations. In addition to the initial state of the retrainprocedure, it is also possible to monitor other internal states of theretrain procedure, and perform a "kick" according to the invention if amodem is "stuck" in these states.

APPENDIX

The following is an extract from pages 14-16 of ITU-T Recommendationv.32 bis which describes the start-up procedure of a v.32 bis connectionbetween modems, and the retrain procedure, which are referred to in thebody of the specification of this document.

6. Start-up procedure

The procedure for achieving synchronism between the calling modem andthe answering modem on international GSTN connections is shown in FIG.3/V.32 bis. The procedure includes the estimating of round-trip delayfrom each modem, the training of echo cancellers and receivers initiallywith half-duplex transmissions, and the exchanging of rate signals forautomatic bit-rate and mode selection.

6.1 Call mode modem

After receiving the answer tone for a period of at least 1 s asspecified in Recommendation V.25, the modem shall be connected to line(see Note 1 below) and shall condition the scrambler and descrambler inaccordance with § 4.1.

The modem shall repetitively transmit carrier state A as shown in FIG.2-5/V.32 bis.

The modem shall be conditioned to detect (see Note 2 below) one of twoincoming tones at frequencies 600±7 Hz and 3000±Hz, and subsequently todetect a phase reversal in that tone.

On detection of one such phase reversal, the modem shall be conditionedto detect a second phase reversal in the same tone, start acounter/timer and change to repetitively transmitting state C as shownin FIG. 2-5/V.32 bis. The time delay between the reception of this phasereversal at the line terminals and the transmitted AA to CC transitionappearing at the line terminals shall be 64±2 symbol periods.

On detection of a second phase reversal in the same incoming tone, themodem shall stop the counter/timer and cease transmitting.

When the modem detects an incoming S sequence (see § 5.2), it shallproceed to train its receiver, and then seek to detect at least twoconsecutive identical 16-bit rate sequences as defined in Table 5/V.32bis.

On detection of the rate signal (R1), the modem shall transmit an Ssequence for a period NT already estimated by the counter/timer.

After this period has expired (see Note 3 below), the modem shalltransmit the receiver conditioning signal as defined in § 5.2, startingwith an S sequence for 256 symbol intervals.

Transmission of the TRN segment of the receiver conditioning signal maybe extended in order to ensure a satisfactory level of echo cancellation(see Note 4 below).

After the TRN segment, the modem shall apply ON condition to circuit 107and transmit a rate signal (R2) in accordance with § 5.3 to indicate thecurrently available data rates. R2 shall exclude rates not appearing inthe previously received rate signal R1. It is recommended that R2 takealso account of the likely receiver performance with the particular GSTNconnection. It appears that satisfactory performance cannot be attainedat any of the available data rates, then R2 should be used to call for aGSTN cleardown in accordance with Table 5/V.32 bis.

Transmission of R2 shall continue until an incoming rate signal R3 isdetected. The modem shall then, after completing its current 16-bit ratesequence, transmit a single 16-bit sequence E in accordance with § 5.3.2indicating the data rate called for in R3. If, however, R3 is callingfor GSTN cleardown in accordance with Table 5/V.32 bis, then the callmodem shall disconnect from time and effect a cleardown.

The modem shall then transmit continuous scrambled binary ones at thedata rate called for in R3. If trellis coding is to be used, the initialstates of the delay elements of the convolution encoder shown inFIG.1/V.32 bis shall be set to zero.

On detecting an incoming 16-bit E sequence as defined in § 5.3.2, themodem shall condition itself to receive data at the rate indicated bythe incoming E sequence. After a delay of 128 symbol intervals, it shallapply an ON condition to circuit 109, and unclamp circuit 104.

The modem shall then enable circuit 106 to respond to the condition ofcircuit 105 and be ready to transmit data.

6.2 Answer mode modem

On connection to line, the modem shall condition the scrambler anddescrambler in accordance with § 4.1, and transmit the RecommendationV.25 answer sequence. Means, defined in Recommendation V.25 of disablingnetwork cancellers and/or truncating the answer tone may be employed.

After the Recommendation V.25 answer sequence, the modem shall transmitalternate carrier states A and C as shown in FIG. 2-5/V.32 bis.

After alternate states A and C have been transmitted for an even numberof symbol intervals greater than or equal to 128 and an incoming tonehas been detected at 1800±7 Hz for 64 symbol periods (see Note 5 below),the modem shall be conditioned to detect a phase reversal in theincoming tone, start a counter/timer, and change to transmittingalternate carrier states C and A for an even number of symbol intervals.

On detecting a phase reversal in the incoming tone, the modem shall stopthe counter/timer and, after transmitting a state A, revert totransmitting alternate states A and C. The time delay between thereception of this phase reversal at the line terminals and thetransmitted CA to AC transition appearing at the line terminals shall be64±2 symbol periods.

When an amplitude drop is detected in the incoming tone, the modem shallcease transmitting for a period of 16 symbol intervals and then (seeNote 3 below) transmit the receiver conditioning signal as defined in §5.2.

Transmission of the TRN segment of the receiver conditioning signal maybe extended in order to ensure a satisfactory level of echo cancellation(see Note 4 below).

After the TRN segment, the modem shall transmit a rate signal (R1) inaccordance with §5.3 to indicate the data rates currently available inthe answer modem and associated DTE.

On detection of an incoming S sequence, the modem shall ceasetransmitting.

The modem shall wait for a period MT already estimated by thecounter/timer and then, if an incoming S sequence persists, or when an Ssequence reappears (see Note 3 below), the modem shall proceed to trainits receiver.

After training its receiver, the modem shall seek to detect at least twoconsecutive identical incoming 16-bit rate sequences as defined in §5.3.

On detection of a rate signal (R2), the modem shall apply ON conditionto circuit 107 and transmit a second receiver conditioning signal asdefined in § 5.2.

After the TRN segment, the modem shall transmit a second rate signal(R3) in order to indicate the data rate to be used by both modems. Thedata rate selected by R3 shall be within those indicated by R2. It isrecommended that R3 take also account of the likely performance of theanswer modem receiver with the particular GSTN connection established.If R2 is calling for a GSTN cleardown (see Table 5/V.32 bis) and/or ifit appears that satisfactory performance cannot be attained by theanswer modem at any of the available data rates, then R3 should call fora GSTN cleardown, in accordance with Table 5/V.32 bis (see Note 6below).

When the modem detects an incoming 16-bit E sequence as defined in §5.3.2, it shall condition itself to receive data at the rate indicatedby the E sequence.

The modem shall complete the current 16-bit rate sequence and thentransmit a single 16-bit E sequence indicating the data rate to be usedin the subsequent transmission of scrambled binary ones. If trelliscoding is to be used, then the initial states of the delay elements ofthe convolution encoder shown in FIG. 1/V.32 bis shall be set to zero.

The modem shall transmit scrambled binary ones for 128 symbol intervals,then enable circuit 106 to respond to the condition of circuit 105 andbe ready to transmit data.

The modem shall also apply ON condition to circuit 109 and unclampcircuit 104.

Notes to § 6

Note 1--Once an incoming tone is detected at 600±7 Hz or 3000±7 Hz, thecalling modem may proceed with the start-up sequence even if no 2100 Hztone has been detected.

Note 2--In some cases, the incoming tones may be preceded by a specialpattern which may last up to 3100 ms.

Note 3--The TRN segment in the receiver conditioning signal is suitablefor training the echo canceller in the transmitting modem.Alternatively, it is acceptable to proceed the receiver conditioningsignal by A sequence which can be used specially for training the echocanceller, but which need not be defined in detail in theRecommendation. The echo cancellation sequence (if used) must maintainenergy transmitted to line to hold network echo control devices disabled(as required). In order to avoid confusion with Segments 1 or 2 of thereceiver conditioning signal defined in § 5.2, the echo cancellationsequence shall produce a transmitted signal such that the sum of itspower in the three 200 Hz bands centered at 600 Hz, 1800 Hz and 3000 Hzis at least 1 dB less than its power in the remaining bandwidth. Thisapplies for the relative power averaged over any 6 ms time interval. Theduration of this signal must not exceed 8192 symbol intervals.

Note 4--Manufactures are cautioned that a period of 650 ms is needed fortraining any network echo cancellers conforming to Recommendation G.165,that may be encountered on GSTN connections.

Note 5--The answering modem may disconnect from the line if the 1800±7Hz tone is not detected following transmission of the segment AC.However, to assure compatibility with manual originating data stations,it shall not disconnect for at least 3 seconds after the segment AC hasbeen transmitted.

Note 6--If R3 is calling for a GSTN cleardown, the modem shall repeatthe transmission of signal R3 for not less than 64 symbol intervalsbefore clearing the connection.

7. Retrain procedure

A retrain may be initiated during data transmission if either modemincorporates a means of detecting unsatisfactory signal reception FIG.4a)/V.32 bis shows a retrain event initiated by the calling modem andFIG. 4b)/V.32 bis shows a retrain event initiated by the answeringmodem. The procedure is as follows:

7.1 Call mode modem

Following detection of unsatisfactory signal reception or detection ofone of two tones at frequencies 600±7 Hz and 3000±7 Hz for more than 128symbol intervals, the modem shall turn OFF circuit 106 clamp circuit 104to binary one and repetitively transmit carrier state A as shown in FIG.2-5/V.32 bis. It shall then proceed in accordance with § 6.1 beginningwith the third paragraph.

7.2 Answer mode modem

Following decision of unsatisfactory signal reception or detection of atone of frequency 1800±7 Hz for more than 128 symbol intervals the modemshall turn OFF circuit 106, clamp circuit 104 to binary one and transmitalternate carrier states A and C for an even number of symbol intervalsnot less than 128. It shall then proceed in accordance with § 6.2beginning with the third paragraph.

7.3 Operation of circuits 107 and 109 during retrain procedure

Circuit 107 shall be maintained in the ON condition during the retrainprocedure.

Circuit 109 shall be maintained in the ON condition except that the OFFcondition may optionally be applied if transmission of the AA segment inthe call modem or the first AC segments in the answer modem continuesfor a period exceeding 45 seconds. If the retrain procedure issubsequently completed, the ON condition shall be re-applied to circuit109 at the time that circuit 104 is unclamped.

I claim:
 1. A method for controlling a wireless connection between twomodems in the event of disturbance, comprising:initiating a retrainprocedure in the event of disturbance of a connection between the modemswhen either one of the modems receives a signal of unsatisfactoryquality, in which retrain procedure the modems transmit predeterminedretrain signals to each other in order to achieve synchronism betweenthem monitoring progress of the retrain procedure, and every time theprocedure is stuck in a state other than a data transmission state for aperiod of time longer than a predetermined limit, commanding therespective modem to initiate a repetition of a start-up procedure bymeans of which the connection between the modems was originallyestablished.
 2. The method according to claim 1, wherein:in practicingsaid commanding, if the number of commanded start-up procedures exceedsa predetermined limit, entirely disconnecting the connection between themodems.
 3. The method according to claim 1, wherein:in practicing saidcommanding, if the total time the procedure remains in a state otherthan a data transmission state exceeds a predetermined limit, entirelydisconnecting the connection between the modems.
 4. The method accordingto claim 1, wherein:said monitoring progress of the retrain procedureincludes controlling an initial state of the retrain procedure, which isa state other than a data transmission state.